Combustors, such as those used in gas turbines, for example, mix compressed air with fuel and expel high temperature, high pressure combustion gas downstream. The energy stored in the gas is then converted to work as the high temperature, high pressure combustion gas expands in a turbine, for example, thereby turning a shaft to drive attached devices, such as an electric generator to generate electricity. The shaft has a plurality of turbine blades shaped such that the expanding hot gas creates a pressure imbalance as it travels from the leading edge to the trailing edge, thereby turning the turbine blades to rotate the shaft.
FIG. 1 shows a gas turbine 20. Air to be supplied to the combustor 10 is received through air intake section 30 of the gas turbine 20 and is compressed in compression section 40. The compressed air is then supplied to headend 50 through air path 60. The air is mixed with fuel and combusted at the tip of nozzles 70 and the resulting high temperature, high pressure gas is supplied downstream. In the exemplary embodiment shown in FIG. 1, the resulting gas is supplied to turbine section 80 where the energy of the gas is converted to work by turning shaft 90 connected to turbine blades 95.
One effective method of cooling the turbine blade exposed to very high gaspath temperatures is to generate serpentine cooling passages within the blade. The resulting internal cooling circuit channels coolant, normally extracted from the compressor bleed, through the airfoil of the blade and through various film cooling holes around the surface thereof. One type of airfoil extends from a root at a blade platform (not shown), which defines the radial inner flowpath for the combustion gases, to a radial outer cap or blade tip section, and includes opposite pressure and suction sides extending axially from leading to trailing edges of the airfoil. The cooling circuit extends inside the airfoil between the pressure and suction sides and is bounded at its top by the blade tip section. As coolant flows through the cooling passages, heat is extracted from the blade, thereby cooling the part.
FIG. 2A is a cross sectional view of a serpentine cooled turbine blade 95 with a conventional squealer tip design. FIG. 2B is a cross sectional view along lines A-A of FIG. 2A. As shown, squealer tip 100 has squealer tip floor 110. As the coolant flows through the cooling circuit defined by serpentine walls 130, the heat accumulated on the turbine blade 95 are transferred to the coolant, and the heated air is expelled through openings on the trailing edge 140.
However, the trailing edge tip region of a serpentine cooled turbine blade is subjected to very high heat loads as, due to gas path migration effects, hot gas originating from the leading edge mid-span surrounds the region on the pressure side of the blade. These high heat loads cause very high coating/metal temperatures that can lead to premature coating failure and substrate oxidation. Because thermal barrier coating, also known as TBC, is generally removed locally at the tip after the first rub, it is of limited benefit. Furthermore, adding film holes in this region is of limited cooling benefit due to the difficulty in configuring film holes such that they penetrate into the cooling cavities of the blade.